1. Field of Invention
This invention relates to coatings on metals and related methods therefor and, in particular, to coatings such as anodized coatings on metal and metal alloys that exhibit resistance to steam, including superheated steam, and resistance to alkaline and acidic degradation.
2. Discussion of Related Art
Anodic coatings for aluminum and aluminum alloys, for example, are typically classified by type and class. Type I coatings are derived from chromic acid electrolyte and type IB coatings from low voltage processes in chromic acid electrolyte. Type IC coatings are typically produced by non-chromic acid anodizing. Type II coatings are produced in a sulfuric acid electrolyte. Type III coatings, also referred to as hard anodic coatings, are also produced in a sulfuric acid electrolyte. Class 1 coatings are dye free coatings and class 2 coatings are dyed coatings.
Type II and Type III are characterized as having significant porosity by the nature of the cell formation and coatings can be left unsealed or can be sealed. Sealing of anodic coatings on metal surfaces can be classified based on the composition of the seal solution, based on the operating temperature, or based on the mechanism of the process.
Traditional sealing processes can be considered to include hot (boiling) deionized water sealing, steam sealing, sodium or potassium dichromate sealing, sodium silicate sealing, nickel acetate sealing, nickel fluoride sealing, and new sealing processes, such as cobalt acetate sealing, trivalent chromium sulfate or acetate sealing, cerium acetate sealing, zirconium acetate sealing, triethanolamine-based sealing, lithium or magnesium salt-based sealing, potassium permanganate sealing, polymer-based sealing, and oxidizing corrosion inhibitor-based sealing such as those involving molybdate, vanadate, tungstate, and perborate agents.
Sealing processes based on temperature can involve high temperature sealing (above 95° C.) with steam, hot water, and dichromate; mid-temperature sealing (80° C.-95° C.) with silicate and divalent or trivalent metal acetates, triethanolamine-based techniques, and oxidizing corrosion inhibitor based techniques; low temperature sealing (70° C.-80° C.) with metal acetate, and ambient temperature sealing (25° C.-35° C.) with nickel fluoride.
Sealing processes can also be classified by sealing mechanism as by hydrothermal sealing which typically involves converting aluminum oxide to boehmite (aluminum oxide hydroxide, AlO(OH)); physical or chemical impregnation and filling of the micropores of the anodic layer by dichromate, silicate, nickel fluoride, and polymer compounds; electrochemical sealing which involves electrophoretic migration and deposition anionic species in the micropores; and corrosion inhibition sealing which involves thermal motion and diffusion promoted absorption of corrosion inhibitors into the micropores.
Sealing of type I, IB, IC, II, 10B and III coatings can be performed by immersion in aqueous dichromate solutions with a pH of 5-6 and a temperature of 90° C.-100° C. for 15 minutes, by immersion in boiling deionized water, or by immersion in a cobalt acetate solution or a nickel acetate solution. Sealing can also be performed by immersion in a sealing medium of hot aqueous nickel acetate or cobalt acetate with a pH of 5.5-5.8 or by immersion in boiling deionized water. Duplex sealing with hot aqueous solutions of nickel acetate and sodium dichromate can also be performed on type I, IB, IC, II, 10B, and III coatings. In accordance with MIL-A-8625, type III coatings for abrasion or resistance service are typically not sealed. Otherwise, type III coatings can be sealed by immersion in boiling deionized water, in a hot aqueous sodium dichromate solution, or in hot aqueous solution of nickel acetate or cobalt acetate and other sealing mechanisms.
Smutting can be encountered in sealing processes, typically during hydrothermal sealing procedures. Smutting can result from the conversion of the coating surface to boehmite. Smutting is typically associated with high operational temperature and pH, long immersion time, aged sealing solution containing too much dissolved solids and breakdown components of additives, and shortage of anti-smutting agents and/or surface active agents. Anti-smutting agents can inhibit the formation of boehmite on the coating surface without adversely affecting the sealing process within the micropores. Typical anti-smutting agents include, for example, hydroxycarboxylic acids, lignosulphonates, cycloaliphatic or aromatic polycarboxylic acids, naphthalene sulphonic acids, polyacrylic acids, phosphonates, sulphonated phenol, phosphonocarboxylic acids, polyphosphinocarboxylic acids, phosphonic acids, and triazine derivatives.
As illustrated in FIG. 1, anodic coatings 102 on some nonferrous metals such as aluminum and aluminum alloys 104 can have porous structures with cells including pores or voids 106 or micropores and walls of a metal oxide, and a barrier oxide layer 118. The porous structure can be susceptible to aggressive environments and water absorption, which can result in degradation of the anodized layer.
Conventional hydrothermal sealing process is typically performed by immersion or exposure to hot water or steam at temperatures above 80° C. to hydrate the anhydrous oxide (Al2O3) in anodic coatings to form boehmite-like crystals (AlO(OH)) according to the following reaction:Al2O3 (anodic coating)+H2O→2AlO(OH)  (1)
Because boehmite (3.44 g/cm3) has a larger volume per unit mass than aluminum oxide (3.97 g/cm3) and because two moles of boehmite can be formed from one mole of aluminum oxide, the micropores are eventually at least partially filled, and typically blocked and closed by the resultant expansion of the cell walls of the anodic coating during hydrothermal sealing. Hydrolysable salts and organic agents can be utilized to improve the sealing performance and efficiency, saves energy, and minimizes the formation of smut on the surface of anodic coatings. For example, nickel ions from nickel acetate can catalytically hydrate aluminum oxide to boehmite through the co-precipitation of nickel hydroxide (Ni(OH)2):Ni2++2OH−→Ni(OH)2↓  (2)
In dichromate sealing, aluminum oxydichromate (AlOHCrO4) or aluminum oxychromate ((AlO)2CrO4) forms in the micropores according to the reactions:Al2O3+2HCrO4−+H2O→2AlOHCrO4−↓+2OH− (pH<6.0)  (3)Al2O3+HCrO4−→(AlO)2CrO4−+OH− (pH>6.0)  (4)
In silicate sealing, silicate ions react with aluminum oxide to form aluminum silicate (Al2OSiO4) in the micropores of an anodic coating according to the following reaction:Al2O3+SiO32−+H2O→Al2OSiO4−↓+2OH−  (5)
The micropores of an anodic coating are not completely filled and closed in either dichromate sealing or silicate sealing. Accordingly, poor results may be anticipated if an acid dissolution test or a dye stain test is used to evaluate the sealing quality. However, dichromate sealing or silicate sealing actually enhances the corrosion resistance of anodic coatings on aluminum, which is ascribed to the role of chromate or silicate in inhibiting the corrosion of aluminum.
Cold sealing processes typically involve nickel fluoride-based sealing techniques. Because cold sealing processes are typically performed at room temperature, reaction (1) does not normally occur in the micropores and voids of an anodic coating. With the catalytic effect of co-precipitation of nickel hydroxide and aluminum fluoride, aluminum oxide is transformed to aluminum hydroxide instead of boehmite at temperatures below 70° C., as expressed in the following reactions:Ni2++2OH−→Ni(OH)2−  (2)Al2O3+6F−+3H2O→2AlF3−+6OH−  (6)Al2O3+3H2O→2Al(OH)3↓  (7)
As with dichromate and silicate sealing, cold nickel fluoride sealing is an impregnation process that does not completely fill and close the micropores and voids, despite the approximate 150% increase in volume when Al2O3 (3.97 g/cm3) is transformed to Al(OH)3 (2.42 g/cm3) in accordance with reaction (7). It is recognized that aluminum hydroxide is chemically less stable and more soluble in aqueous solutions than boehmite. The formed Al(OH)3 tends to be spongy rather than crystalline in form and the sealed anodic article performs poorly when evaluated with acid dissolution or dye stain tests.
Consequently, the anti-corrosion performance of anodized articles post treated by cold sealing can be considered inferior to that treated with conventional hydrothermal sealing and other impregnation processes mentioned above.